1. Field of the Invention
The present invention relates to a light scattering type smoke sensor for sensing smoke by detecting light scattered by smoke.
2. Description of the Related Art
In a light scattering type smoke sensor of this kind, labyrinth members constitute a smoke detecting chamber which allows smoke to efficiently enter from the outside and cuts off light entering from the outside, and optical axes of light emitting and receiving sections are disposed in such a manner that their optical axes intersect each other in the smoke detecting chamber to detect light scattered by the smoke.
As a light emitting device constituting the light emitting section, such a conventional light scattering type smoke sensor uses an infrared LED (light emitting diode) having a relatively wide directional angle of 30.degree. to 60.degree.. Consequently, the scattering angle at which the optical axes of the light emitting and receiving sections intersect, and the shapes and reflection angles of the labyrinth members have to be designed so that the light receiving section is prevented from directly receiving light of the relatively wide directional angle from the infrared LED.
In the conventional light scattering type smoke sensor, the light emitting device has a relatively wide directional angle. When the sensor is constructed to reduce its thickness, therefore, a part of direct light from the light emitting device and light reflected by the labyrinth members enter the light receiving device, thereby increasing the zero-point level. This produces a problem in that such a sensor cannot be constructed in a thin form.
The zero-point level means an output of the light receiving device obtained when there is no smoke in the smoke detecting chamber. If the light receiving device easily receives reflected light when there is no smoke in the smoke detecting chamber, the zero-point level is naturally increased so as to deteriorate the S/N ratio and reliability.
If the light emitting section may be assembled by placing a lens or the like in front of the light emitting device in order to reduce the directional angle of the light emitting section to thin the sensor, the production cost of the light emitting section is increased and a positional error in assembly between the light emitting device and the lens causes the light beam to be deviated in direction so that the sensor has to be assembled in a highly accurate manner to impair the productivity thereof.
In a light scattering type smoke sensor of this kind, the light receiving section has to receive only light scattered by smoke. Namely, the positions, shapes and reflection angles of the labyrinth members have to be designed so that the light receiving section is prevented from receiving direct light from the light emitting section multi-reflected light which is reflected at a plurality of times by the labyrinth members. However, the conventional light scattering type smoke sensor is designed without paying sufficient consideration on this point, thereby producing a problem in that the zero-point level is increased.
Recently, in the view point of the interior of a room, external appearance of a device or the like, the demand for constructing a sensor of this kind in a thin form is increasing.
In the above-described conventional light scattering type smoke sensor, however, the light emitting device has a relatively wide directional angle. Even when the sensor is constructed so as to have a thin form, therefore, direct light from the light emitting device is vertically reflected by the ceiling and bottom faces of the smoke detecting chamber, and the reflected light and light which is again reflected by these faces and the labyrinth members enter the light receiving device to increase the zero-point level. This produces a problem in that such a sensor cannot be constructing in a thin form. In addition, when the ceiling and bottom faces of the smoke detecting chamber are soiled, the zero-point level is further increased.
To comply with this, a configuration in which a throttle (opening) or a hood is disposed in front of the light emitting device may be employed. However, this configuration has problems in that all the light emitted from the light emitting device cannot effectively be used, and that the cost of the sensor is increased.
The configuration in which a throttle (opening) for reducing the projecting area of the light emitting device is disposed can prevent the zero-point level from being increased. In this configuration, however, all the light emitted from the light emitting device cannot effectively be used, with the result that the signal level due to light scattered by smoke is lowered.